Electrochemical cells comprising solid alkali ion conductive electrolyte membrane that selectively transport alkali ions are known in the art. By having an alkali ion-selective membrane in the electrochemical cell, alkali ions are allowed to pass between the cell's anolyte compartment (or negative electrode compartment) and catholyte compartment (or positive electrode compartment) while other chemicals are maintained in their original compartments. Thus, through the use of an alkali ion-specific membrane, an electrochemical cell can be engineered to be more efficient and to produce different chemical reactions than would otherwise occur without the membrane.
Solid alkali ion conductive electrolyte membranes are used in electrochemical cells for various reasons, including, but not limited to, being ion conductive, ion selective, water impermeable, chemically stable, electronic insulator, and so forth. By way of example, NaSICON (Na Super Ion CONducting) membranes selectively transport sodium cations, while LiSICON (Li Super Ion CONducting) and KSICON (K Super Ion CONducting) membranes selectively transport lithium and potassium cations, respectively. Other examples of solid alkali ion conductive electrolyte membranes include beta alumina, sodium-conductive glasses, etc.
Electrochemical cells comprising solid alkali ion conductive membranes are used to produce a variety of different chemicals and to perform various chemical processes. In some cases, such electrochemical cells convert alkali salts into their corresponding acids. In other cases, such electrochemical cells may also be used to separate alkali metals from mixed alkali salts.
One example of a conventional electrochemical cell 10 is illustrated in FIG. 1. Specifically, FIG. 1 illustrates an electrochemical cell 110 which comprises an negative electrode compartment 112 and a positive electrode compartment 114 that are separated by a NaSICON membrane 116. During operation, the negative electrode compartment 112 comprises an aqueous sodium salt solution (NaX, wherein X comprises an anion capable of combining with a sodium cation to form a salt) and current is passed between an anode 118 and a cathode 120. Additionally, FIG. 1 shows that as the cell 110 operates, water (H2O) is split at the anode 118 to form oxygen gas (O2) and protons (H+) through the reaction 2H2→O2+4H++4e−. FIG. 1 further shows that the sodium salt NaX in the anolyte (or negative electode) solution is split (according to the reaction NaX+H+→HX+Na+) to (a) allow sodium cations (Na+) to be transported through the NaSICON membrane 116 into the positive electrode compartment 114 and (b) to allow anions (X−) to combine with protons to form an acid (HX) that corresponds to the original sodium salt.
The above-mentioned electrochemical cell may be modified for use with other alkali metals and acids corresponding to the alkali salts used in the anolyte solution. Moreover, it will be appreciated that other electrochemical reactions may occur which result in hydroxyl formation and a corresponding rise of pH within the positive electrode compartment 114. High pH catholyte solutions in such electrochemical cells have shortcomings. In one example, at a higher pH, such as a pH greater than about 10, certain alkali ion-conductive ceramic membranes, such as NaSICON membranes, begin to structurally degrade by dissolution. Accordingly, as the electrochemical cell 110 operates and base is produced in the positive electrode compartment 114, the cell 110 becomes less efficient or even inoperable. In another example, base produced in the positive electrode compartment 114 can actually damage the alkali ion conductive membrane, such as a NaSICON membrane, and thereby shorten its useful lifespan.
Batteries are a class of electrochemical cells that are used to store and release electrical energy for a variety of uses. In order to produce electrical energy, batteries typically convert chemical energy directly into electrical energy. Generally, a single battery includes one or more galvanic cells, wherein each of the cells is made of two half-cells that are electrically isolated except through an external circuit. During discharge, electrochemical reduction occurs at the cell's positive electrode, while electrochemical oxidation occurs at the cell's negative electrode. While the positive electrode and the negative electrode in the cell do not physically touch each other, they are generally chemically connected by at least one (or more) ionically conductive and electrically insulative electrolyte(s), which can either be in a solid or a liquid state, or in combination. When an external circuit, or a load, is connected to a terminal that is connected to the negative electrode and to a terminal that is connected to the positive electrode, the battery drives electrons through the external circuit, while ions migrate through the electrolyte.
Batteries can be classified in a variety of manners. For example, batteries that are completely discharged only once are often referred to as primary batteries or primary cells. In contrast, batteries that can be discharged and recharged more than once are often referred to as secondary batteries or secondary cells. The ability of a cell or battery to be charged and discharged multiple times depends on the Faradaic efficiency of each charge and discharge cycle.
Rechargeable batteries based on sodium and lithium can employ a solid primary electrolyte separator, such as a solid alkali ion conductive electrolyte membrane. The principal advantage of using a solid ion conductive electrolyte membrane is that the Faradaic efficiency of the resulting cell approaches 100%. Indeed, in almost all other cell designs, the electrode solutions in the cell are able to intermix over time and, thereby, cause a drop in Faradaic efficiency and loss of battery capacity.
Thus, while molten sodium-based rechargeable batteries are available, these batteries must be operated at temperatures above about 100° C. At such temperatures, the solid alkali ion conductive electrolyte membrane may be exposed to catholyte solutions that are chemically reactive to the alkali ion conductive electrolyte membrane making it susceptible to degradation by dissolution.
In still other conventional batteries, the electrochemical cells may be operated using molten salts which may be chemically reactive to the alkali ion conductive electrolyte membrane.
Thus, while electrochemical cells comprising a positive electrode compartment and an negative electrode compartment that are separated by a solid alkali ion-conductive membrane are known, challenges still exist, including those mentioned above. Accordingly, it would be an improvement in the art to protect the solid alkali ion conductive electrolyte membrane from undesired chemical reactions, including degradation by dissolution, and thereby maintain its structural stability and alkali ion conductivity.